This invention pertains to passive-type range determining systems for measuring the distance (i.e., range) to a source of electromagnetic radiation.
There are certain navigational, surveillance and military operations in which it is desirable to passively determine the range between a monitoring position and a target or object, having a scanning source of electromagnetic radiation, such as emitted by a radar transmitting antenna. One example of such a system is described in a copending U.S. patent application, Ser. No. 080,533 filed Oct. 1, 1979, as a continuation-in-part application of Ser. No. 488,401 filed July 15, 1974, now abandoned, for SERVO-LOOP PROCESSOR, by Philip Jones and Moorfield Storey, Jr. As disclosed therein, signal monitoring equipment carried by a manned or unmanned aircraft, is capable of measuring the range to the emitter target without requiring maneuvers of the monitoring aircraft, which are time consuming and in some circumstances tactically impractical. The range is determined by measuring a small time differential that occurs between the receipt of the target's emitted scanning signal at first and second spaced-apart receiving antennas carried by a monitoring aircraft. In this example, the receiving antennas are mounted on the opposite wing tips of the monitoring aircraft such that a distant source of scanning RF radiation sweeps across the receiving antennas, causing relatively early and late arriving signals to be produced in separate receiving channels. In the above-mentioned patent application, a special, precision servo-loop processor is provided for resolving very small time differentials between the early and late channel signals. An electrical signal representing such time differential is then processed with other separately detected signals representing the scan rate of the source and an angle of arrival of the source radiation with respect to the receiving antennas, to develop a signal representing the range.
While systems of this nature are effective in determining the range to the source of transmission when it is of a scanning type, such systems are not capable of making a range measurement when the source of transmission is fixed or does not scan in a predictable fashion. Since the range determining function as characterized above, depends in part on information defining the rate at which the transmission sweeps across the spaced-apart receiving antennas, the absence of such sweeping or scanning of the source renders the measurement system ineffectual.
Accordingly, an object of the present invention is to provide a range determining method and apparatus capable of measuring the range to a non-scanning source of electromagnetic transmission. A related object is to provide such a range determining method and apparatus in which the source can be either scanning or non-scanning.
Another object of the invention is to provide a passive-type range determining apparatus that has the above noted capability, and which can be carried on board a moving carrier, such as on a manned or unmanned aircraft.
Additionally, the invention has the features of eliminating the need to measure the scan rate of the source of transmission (because the range measurement does not use source scanning information); and for the same reason, the invention does not need to rely on an assumed continuation of a previously measured rate of scanning of the source, which may not be a valid assumption should the rate be continuously changing in some unpredictable fashion during the ranging process.